This invention relates generally to through-air devices (TADs) and more particularly to controlling moisture or other profiles in webs being treated by TADs. As used herein, the term “through-air device” generally refers to a device for drawing a fluid (typically a gas such as heated air, ambient air, combustion products and/or a vapor, although a liquid such as water can be used in some applications) through permeable webs to treat the webs. Thus, the use of the word “air” in “through-air device” is in no way limiting to air. It should be understood that reference to the term “air” hereinafter includes other fluids as well. Common examples of TADs include through-air dryers, bonders and curers. Other applications of TADs include extraction, cooling, moisturizing, washing and porosity measurements.
In many web processing methods, such as paper making, TADs are used for drying the web after, before or instead of pressing devices. Typically, such a TAD incorporates a hollow, rotating roll fitted with a perforated or otherwise permeable shell around which a wet web is partially wrapped as the web is passed through the TAD. The web is often supported on a continuous fabric as it is passed through the TAD. Heated air (gas or vapor) passes through the permeable web, fabric and roll so as to cause drying of the web.
In through-air thermal processes such as drying, the web necessarily serves as a flow resistance. The local magnitude of this resistance can vary as local web properties, such as basis weight and moisture content, vary across the width of the web and thus the flow of the supply air, even when uniformly distributed upstream of the web, can grow non-uniform as it approaches the web. For example, in a drying process, more air can flow through drier, lighter or more-permeable portions of the web, tending to exacerbate existing cross-machine moisture profiles. The problem of inherent web non-uniformity is compounded by the airflow arrangement used in many TADs; that is, the air is typically exhausted through one or both ends of the roll. This introduces an inherent tendency for through-air flow to favor the exhaust side or sides, resulting in diminished drying, bonding or curing rates on the opposite end or center of the web. Means exist to compensate for or correct this flow bias but they require the introduction of pressure losses (i.e., increased energy consumption and production costs.)
In addition, the air delivered to the supply plenum, just upstream of the web's surface, is not always distributed uniformly with respect to both temperature and air speed. Non-uniformity can result from such things as poor mixing upstream or thermal loss. Thermal loss through duct walls tends to depress the supply air temperature on both sides of the supply plenum while air speed can be expected to decrease near the plenum walls. Thus, there exists a tendency to under treat the outermost edges of the web. Any non-uniformity in supply and exhaust air density (due to temperature and/or air speed variation) can result in the development of a cross-machine pressure gradient within the gap between the supply plenum and surface of the TAD roll. There can thus be a tendency for supply air to “blow-out” from within this gap into the machine room or for ambient air to be sucked into the gap.
Furthermore, when threading a production line, the web is typically first introduced to the TAD as a narrow strip (referred to as the tail) which occupies only a fraction of the full production width. This means that supply air tends to flow around the web through that portion of the TAD roll's surface offering the least resistance resulting in ineffective thermal treatment of the tail and the tail not being properly secured on the surface of the roll. It is desirable to process (e.g., dry or bond) the tail as the integrity (strength) of the tail is increased, thereby making any handling operations downstream of the TAD easier and more efficient. The treading process through the TAD is less problematic and more secure when the tail is firmly held to the roll surface.
TADs currently rely on profiling devices, installed within the TAD roll, to eliminate cross-machine flow non-uniformity due to duct configuration. Web non-uniformity resulting from such causes as varying web characteristics, supply and gap pressure imbalance, and transients, such as threading, has generally not been addressed. Typical control devices consist of perforated tubes, mounted within the roll, that offer either a varying flow resistance (smaller or fewer perforations approaching the exhaust end or ends of the roll) or a resistance that substantially exceeds or overpowers that due to the web itself. In both instances, system pressure loss due to the profiling device can be large. In neither approach can the resistance be easily reduced or increased or otherwise adjusted to suit the specific conditions obtained when producing a given web. The devices are thus typically sized for worst-case operating scenarios such that much of the pressure loss associated with their use can be considered parasitic when producing off-design webs.
Accordingly, there is a need for a TAD that can accommodate inherent upstream, cross-machine variation in web characteristics, as well as supply air non-uniformity within the TAD, to produce webs exhibiting more uniform treatment (such as moisture, bonding or curing profiles) downstream of the TAD.